The present invention relates generally to methods of stabilizing a subterranean formation. In particular, the present invention relates to methods of stabilizing swellable clays in the formation and methods of reducing the formation of sludge in the formation using a biopolymer based cationic surfactant.
The recovery of fluids such as oil and gas from subterranean formations has been troublesome in formations that contain water sensitive minerals, e.g., water-swellable clays such as clays in the smectite group and fines capable of migrating when disturbed such as silica, iron minerals and alkaline earth metal carbonates. Clay minerals ubiquitous in sedimentary rocks are hydrous aluminum phyllosilicates characterized by a stack of two-dimensional sheets composed of the SiO4 tetrahedron and/or the AlO4 octahedron. They can cause major problems during hydrocarbon exploration as clay minerals have a tendency to swell/migrate in the presence of water. The water can come from injection fluids, drilling muds, stimulation fluids, and gels. The ion-exchangeable cations present in-between the aluminosilicate sheets attract water in the interlayer molecular spaces and cause swelling. For example, the smectite group of clays such as montmorillonite absorbs water and expands in volume. During oil exploration, introduction of water causes these clays to swell, ultimately degrading the porosity and reducing the formation permeability. The clay mineral kaolinite, on the other hand, generates fines in the presence of water and tends to migrate. These fines migrate through the formation, blocking the pore spaces and reducing the permeability of the formation.
This loss in permeability and plugging or impairing the flow of formation fluids toward the well bore results in either a loss of the formation fluids or a decrease in the rate of recovery from the well. Sometimes the migrating fines are produced with the formation fluids and present abrasion and other problems with the above-ground equipment.
In an effort to overcome these problems, various methods have been developed for treating subterranean formations to stabilize swelling clays and migratable fines. For example, it has been a common practice to add salts to aqueous drilling fluids. The salts adsorb to clay surfaces in a cation exchange process and can effectively reduce the swelling and/or migration of the clays. Various polymers and consolidating resins have also been used. However, in many areas, environmental regulations restrict the use of high salt concentrations and various resin and polymer compositions.
Various acids are used to stimulate subterranean hydrocarbon-bearing formations. The acid functions to dissolve acid soluble materials in the formation so as to increase the permeability of the formation. The permeability increase is effected by cleaning or enlarging flow channels leading to the well bore allowing more oil or gas to flow to the well bore.
Another problem is encountered when acid is introduced into the formation. Hydrochloric acid (HCl), particularly when at high concentrations of about 15% and greater, can cause the development of sludge when the acid is placed in contact with certain types of crude oil. The sludge formation problem is exacerbated when the acid that is in contact with the crude oil also contains ferric ion.
Certain crude oils contained in subterranean formations produce sludge upon contact with aqueous acid solutions during the carrying out of acidizing treatments. The sludge formed is an asphalt-like material which precipitates in the formations and often plugs or clogs the enlarged flow channels formed therein. Interaction studies between sludging crude oils and acids have shown that precipitated solids or films are formed at the acid oil interface. The precipitates are mainly asphaltenes, resins, paraffins and other high-molecular weight hydrocarbons.
When sludges are produced in crude oil, the viscosity of the oil drastically increases. Due to this increase, the rheological characteristics of the fluid can exhibit negative effects by a dramatic decrease in formation fluid-drainage properties. The treated formations are very slow to clean up, if at all, and often the acidizing treatments produce a decrease in permeability and reduction in oil production instead of an increase.
Methods for preventing or reducing sludge formation include the addition of alkyl phenols, fatty acids, surfactants and the like to stabilize the acid-oil emulsion against sludge formation. The various materials suffer from various problems during use. The anti-sludge agents may be incompatible with other additives such as corrosion inhibitors used in the acid, suffer from poor dispersibility in acids or brines, or be corrosive to metals and the like.
Thus, there is a continuing need for improved methods and compositions for treating subterranean formations. Specifically, there is a need for improved methods and compositions for preventing the swelling of clays and reducing the formation of sludge in oil and gas operations.